The present invention relates to processes for producing gasoline.
Gasoline is extensively used in the car industry.
Known in the art are some processes for producing gasolines by catalytic cracking of a heavy petroleum feedstock such as bunker fuel, gas oil, vacuum distillate using an amorphous or zeolite-containing aluminosilicate catalyst, followed by regeneration of the coked catalyst and recycle of the regenerated catalyst to the cracking stage. Produced from such cracking is a reaction mixture containing gasoline which is then separated from the reaction mixture by rectification (cf. C. W. Strouther, Oil and Gas Journal, 1972, v. 70, No. 2, pp. 102-104, 106-110; Hydrocarbon Processing, 1972, v. 51, No. 9; Hydrocarbon Processing, 1968, v. 47, No. 9, p. 148).
Gasolines produced by these prior art processes have an octane number, as it is, of 79 - 82 points by the motor-method and 89 - 93 by the research method. In some cases, depending on the starting stock quality, gasolines may be produced with an octane number per se of up to 84 points by the motor-method and as high as 96 by the research method.
Said prior art processes have, however, a disadvantage residing in the fact that the gasolines produced cannot be used, individually, as top grade automobile gasolines. To be used for this purpose, they should be compounded with tetraethyl lead and a high-octane component.
Also known in the art is a process for producing gasoline by catalytic cracking of a heavy petroleum feedstock (cf. U.S. Pat. No. 3,394,076 and U.S. Pat. No. 3,448,037).
In this process, a heavy petroleum feedstock such as a straight-run gas oil is subjected to cracking in the presence of a regenerated zeolite-containing aluminosilicate catalyst which is in a state of an ascending flow (first reaction stream) at a temperature of, for example, from 471.degree. to 524.degree. C and at a space-mass velocity of the feed of 40 to 65 hr.sup.-1. The reaction products along with the coked catalyst are delivered to a settling zone of the reactor, wherein the reaction products are separated from the coked catalyst.
The reaction products contain a fraction with a specific gravity of from 0.75 to 0.95 which incorporates gasoline, and a recycled fraction boiling above 221.degree. C which are recovered by rectification.
The separated recycled fraction is again subjected to catalytic cracking in the presence of said catalyst which is in a state of an ascending flow (second reaction stream) at a temperature of from 485.degree. to 537.degree. C at a mass-space velocity of the recycled fraction of 40 to 65 hr.sup.-1. The reaction products containing gasoline along with the coked catalyst are delivered to the same settling zone of the reactor, wherein the reaction products resulting from cracking of the first reaction stream are separated.
In the settling zone the reaction products are compounded, separated from the coked catalyst and delivered to separation, while the coked catalyst is fed as a combined stream to the regeneration.
This process results in 65 vol.% of gasoline with the final boiling point of 221.degree. C which corresponds to a yield of 45 wt. % with the final boiling point of 195.degree. C.
This prior art process has a disadvantage residing in that the resulting gasoline, as is, has an insufficient octane number, i.e. 80.1 points by the motor method and 92.5 points by the research method.
The resulting gasoline may be used as a top grade commercial gasoline only after addition of tetraethyl lead and high-octane components. However, ethylated gasoline produced by this method causes atmospheric pollution with lead compounds entrained with exhausted gases.
Another disadvantage of this prior art process resides in that the second reaction stream -- the fraction with a high final boiling point facilitates, under the cracking conditions, an extensive coke-formation and hinders the controlled conversion of reactive hydrocarbons.
Still another disadvantage of this prior art process resides in the fact that cracking in both reaction streams is effected at high weight velocities of the feedstock and, hence at low concentrations of the catalyst which does not make it possible to obtain gasoline with the required high chemical stability without adding oxidation inhibitors.
Furthermore, due to the fact that the reaction products of both reaction streams are compounded prior to the separation of gasoline and recycled fraction, the latter fraction which is a feedstock for the second reaction stream is enriched with aromatic hydrocarbons owing to secondary reactions. This results in a higher coke deposition rate on the catalyst and in impaired quality of the resulting gasoline.